Chapter 40 / The Brain Stem 993
Figure 40–7 The pupillary response to light is mediated by
parasympathetic innervation of the iris.Retinal ganglion cells
that contain the photopigment melanopsin act as luminance
detectors, sending their axons through the optic tract to the oli-
vary pretectal nucleus, at the junction of the midbrain and the
thalamus. Neurons in this nucleus project through the posterior
commissure to parasympathetic preganglionic neurons in and
around the Edinger-Westphal nucleus. The axons of the pregan-
glionic cells exit with the oculomotor (III) nerve and contact cili-
ary ganglion cells, which control the pupilloconstrictor muscle
in the iris. (Abbreviations: LGN, lateral geniculate nucleus;
MLF, medial longitudinal fasciculus.)
外侧
膝状体
内侧纵束
橄榄顶盖前核
睫状
神经节
虹膜瞳孔收缩肌
视网膜
神经节细胞
N. III
动眼神经副核
retinal ganglion cells also project to the suprachias-
matic nucleus of the hypothalamus, where they entrain
circadian rhythms to the day–night cycle (Chapter 44).
Vestibulo-ocular reflexes stabilize the image on the
retina during head movement by rotating the eyeballs
counter to the rotation of the head. These reflexes are
activated by pathways from the vestibular ganglion
and nerve to the medial, superior, and lateral vestibu-
lar nuclei, and from there to neurons in the reticular
formation and ocular motor nuclei that coordinate
eye movements. The reflex movements are seen most
clearly in comatose patients, in whom turning the head
will elicit counter-rotational movements of the eyes
(so-called doll’s eye movements). Damage to these
pathways in the pons impairs these movements.
The corneal reflex involves closure of both eyelids as
well as upward turning of the eyes (Bell phenomenon)
when the cornea is gently stimulated (eg, with a wisp
of cotton). The sensory axons from the first division of
the trigeminal nerve terminate in the spinal trigeminal
nucleus, which relays the sensory signals to pattern
generator neurons in the reticular formation adjacent
to the facial motor nucleus. The pattern generator neu-
rons provide bilateral inputs to the motor neurons that
protect the cornea from damage by causing the orbicu-
laris oculi muscle to close the eyelid and the oculomo-
tor nuclei to roll the eyes upward and back in the orbit.
Because the output of the pattern generator is bilateral,
damage along the sensory pathway prevents the reflex
in both eyes, whereas damage to the facial nerve pre-
vents closure on the same side only.
The stapedial reflex contracts the stapedius muscle
in response to a loud sound, thus damping movement
of the ossicles. The sensory pathway is through the
cochlear nerve and nucleus to the reticular formation
adjacent to the facial motor nucleus and from there to
the stapedial motor neurons, which run in the facial
nerve. As described earlier, in patients with injury to
the facial nerve (eg, Bell palsy), the stapedial reflex is
impaired, and the patient complains that sounds in
that ear have a “booming” quality (hyperacusis).
A variety of gastrointestinal reflexes are con-
trolled by multisynaptic brain stem relays. For exam-
ple, the tasting of food causes neurons in the solitary
nucleus that project to the reticular formation adja-
cent to the motor facial and dorsal motor vagal nuclei
to stimulate the preganglionic salivary neurons. The
contact of food in the mouth can also elicit gastric
contractions and acid secretion, presumably through
inputs from the solitary nucleus directly to parasym-
pathetic preganglionic gastric neurons in the dorsal
motor vagal nucleus. In patients who have had Bell
palsy, the damaged VII nerve parasympathetic axons
may regrow aberrantly so that salivary axons reach
the lacrimal gland in error, causing tasty food to initi-
ate reflex tearing (crocodile tears).
The gag reflex protects the airway in response to
stimulation of the posterior oropharynx. The affer-
ent sensory fibers in the glossopharyngeal and vagus
nerves terminate in the spinal trigeminal nucleus,
whose axons project to the reticular formation adja-
cent to the nucleus ambiguus. Branchial motor neu-
rons in the nucleus ambiguus innervate the posterior
pharyngeal muscles, resulting in elevation of the
Kandel-Ch40_0975-1009.indd 993 12/12/20 3:24 PM